You may worry about your waistline or pack-a-day smoking habit, but psychologists say there is a less recognized yet significant health hazard facing Americans: loneliness.

They could have more friends than ever online but, on average, Americans have fewer intimates to confide in than they did a decade ago, according to one study. Another found that 20 percent of all individuals are, at any given time, unhappy because of social isolation, according to University of Chicago psychologist John Cacioppo. And, frankly, they'd rather not talk about it.

"People come into my office and say, 'I'm depressed or obsessive.' They don't say, 'I'm lonely,' " said Jacqueline Olds, a psychiatrist who teaches at Harvard Medical School and co-authored "The Lonely American: Drifting Apart in the Twenty-First Century." "People are so embarrassed about being lonely that no one admits it. Loneliness is stigmatized, even though everyone feels it at one time or another."

Olds wrote the book with her husband, Dr. Richard Schwartz, because, she said, she wanted to bring loneliness "out of the closet." The two were struck by findings from the General Social Survey (conducted by the National Opinion Research Center at the University of Chicago), showing that people reported having fewer intimate friends in 2004 than they had in 1985. When asked how many people they could confide in, the average number declined over that same time period from three to two. In 2004, almost a quarter of those surveyed said they had no one to discuss important matters with in the past six months; in 1985, only 7 percent were devoid of close confidantes.

"Loneliness has a terrible reputation in this country," Olds said. "It's a problem not just with a few people without social skills. It's not synonymous with being a loser."

Independent, busy people

Why are we growing lonelier? Olds said it's partly due to the American notion of independence that makes people not want to appear needy. They may feel alone, but they assume neighbors and friends are similarly busy and wouldn't want to be bothered.

She also points to what she calls "the cult of busyness." In an era of frantic pace and multitasking, people feel they should always be accomplishing something. They work long hours and then, in their limited spare time, they work more - catching up on e-mail, doing the laundry, going to the gym. Socializing often comes last.

But humans are not wired to live alone, researchers say. The impulse for social connection - though it is stronger in some people than others - is rooted in the basic urge to survive. The need is so great, says Cacioppo, that it is reflected in our neural wiring. Most neuroscientists agree, he said, that it was the need to process social cues that led to the expansion of the cortical mantle of the brain.

In "Loneliness: Human Nature and the Need for Social Connection," which he co-authored last year, he wrote, "In other words, it was the need to deal with other people that, in large part, made us who and what we are today."

Loneliness, Cacioppo explained in an interview, has more in common with hunger, thirst and pain than it does with mental illness. It signals that something is wrong and needs to be corrected.

Study of loneliness

The study of loneliness isn't new. David Riesman's "The Lonely Crowd," published in the early 1950s, raised questions about conformity and individuality following the industrial revolution. Riesman described how the growing middle class in America was increasingly populated by an emerging personality, which he called the "other-directed," a type that thrived in the corporate world. Other-directed people wanted to be well-liked, but formed superficial intimacies. They were, essentially, living alone in a crowd.

And psychologist Robert Putnam wrote in his 2000 book "Bowling Alone" about how Americans were growing more disconnected from not only family and friends but also from institutions - churches, civic groups, even bowling leagues - that used to afford community.

More recently, psychologists and other researchers have debated whether the Internet has made us lonelier. Some argue that access to online social networks helps people stay connected, particularly those who are physically isolated by disabilities or age. Others believe that having dozens of friends on social-networking Web sites like Facebook is not an alternative to real-life contact - and it creates a false sense of intimacy.

Researchers also recently have been examining the physical effects of loneliness. With the help of high-tech tools, neuroscientists now can examine how isolation affects the brain. Functional MRI's - those that allow a view of the live brain in action - show that lonely people have different neural responses to viewing positive images of other people.

Cacioppo's research has shown that lonely people have more "micro-awakenings" during sleep, leading to greater fatigue, which in turn can affect cognitive thinking. Other studies have found that people who feel lonely report more sources of stress in their lives, which can affect long-term health. Chronic stress is associated with higher risk of cardiovascular disease and immune system disorders.

How much contact?

What scientists can't tell you is exactly how much social contact you need. Most people pass through lonely phases at some point. So how can you tell if you're at risk? Psychologists have used an assessment called the UCLA Loneliness Scale, which consists of 20 questions, all dealing with perceived feelings of loneliness - from inquiries about feeling "in tune" with others to whether relationships seem meaningful or provide companionship and understanding.

People vary in how they perceive loneliness, Cacioppo said, depending on their need for connection. How a person experiences isolation is partly due to inherited temperament and partly to experience, he said. Some people are more sensitive to social isolation and therefore experience more stress from loneliness. Others may have less need for intimacy and may not suffer from having few intimate friends.

Often it's not the person who lives alone who's the loneliest. In fact, being alone is distinct from feeling lonely. Psychologists say that people with kids, partners and demanding jobs can feel isolated when they fail to heed their own needs for connection.

Shebana Siegel, a San Francisco mother of two and director of philanthropic giving for Jewish Family and Children's Services, said she has to consciously keep in contact with friends. She uses her lunch hour to call friends and arrange meetings or walks with them.

"People are so busy and we're so busy," she said. "If you don't make the effort, you won't see your friends. It's the American way of life. I have friends from Brazil who come here and say they're so lonely."

Cacioppo's book offers a four-part plan (called EASE), although he concedes the steps are not easy. Loneliness can add to depression and decrease motivation to make connections. Cacioppo's plan includes extending yourself, making an action plan, selecting quality relationships and expecting the best from them.

He and others say that perhaps there is bit of an upside to the current economic crisis - that people will rediscover intimacy.

"People can't go out and they have to be home together," Cacioppo said. "It's nice to be able to depend on one another. If you're so wealthy that you don't have to, there is something you're missing."

How lonely are you?

These questions are among the 20 on the UCLA Loneliness Scale, the most widely used tool to measure feelings of social isolation. The scale was developed in 1978, and revised in 1996.

How often do you feel you lack companionship?

How often you do you feel that there is no one you can turn to?

How often do you feel that no one really knows you well?

How often do you feel that you have a lot in common with the people around you?

How often do you feel that there are people who really understand you?

How often do you feel that people are around you but not with you?

How often do you feel shy?

How often do you feel left out?

Getting connected

From Shakespeare to psychiatrists at Harvard Medical School, writers and researchers have considered loneliness and its implications for individuals, society and health. Here are a few of the titles:

"The Lonely Crowd: A Study of the Changing American Character," by David Riesman, Nathan Glazer and Reuel Denney (Yale University Press; $17.95 paperback)

"Bowling Alone: The Collapse and Revival of American Community," by Robert D. Putnam (Simon & Schuster; $16 paperback)

"Loneliness: Human Nature and the Need for Social Connection," by John T. Cacioppo and William Patrick (W.W. Norton & Co.; $25.95)